fs: return null error on readFile() success
fs: return null error on readFile() success

This commit ensures that readFile() callsback with a null
error consistently on success.

PR-URL: https://github.com/nodejs/node/pull/3740
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>
Reviewed-By: Evan Lucas <ad0310dc5270cefb6ee6879263270aed9b1e9982@me.com>
Reviewed-By: Sakthipriyan Vairamani <b84950071fc99921a72f4fa56f6cad2496b3223c@gmail.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node